1. Field of Invention
The present invention relates to a screwdriver bit, especially to a screwdriver bit in which the structural strength is improved and the wearing during operation is reduced.
2. Description of Related Arts
Refer to U.S. Pat. No. 5,868,047, an insert bit for use with a powered screwdriver is revealed. An insert bit includes a shank end, a tip end, and a midportion located between the shank end and the tip end. The shank end is insertable into a driver. The tip end is breakable at a first torque while the midportion is breakable at a second torque. The insert bit features on that the midportion is sized so that the second torque is approximately 20% greater than the first torque. The preselected length of the midportion is approximately 18 mm to 23.5 mm while the preselected diameter of the midportion is approximately 3.55 mm to approximately 6.35 mm. The preselected length and preselected diameter of the midportion is selected to permit the shank end to be rotated at least 14 degrees with respect to the tip end without permanent deformation of the impact bit. The insert bit is austempered and has a desired hardness of from 52-56 HRC.
In the above structure, the shank end of the midportion is an arc taper so as to allow the torsional shear stress originally acting on the tip end to act on the shank end of the midportion. Thus the shank end of the midportion has torsional deformation so as to prolong deformation and breakdown time of the whole insert bit.
However, the above structure has following shortcoming. The length of the shank end is larger than the maximum diameter of the tip end of the cross-point-type wings. The longer the length of the shank end, the larger shear stress thereon. When the shear stress reaches the yield point, the shank end breaks. Being driven by high speed and high torque power tools, the screwdriver bit is unable to disperse the torsional stress due to excessive torsional deformation. This is because that the length of the shank end is too long so that severe vibration occurs on the connection between a driving end of the screwdriver bit and the fastener. Thus too much shear stress acting on the shank end results in breakage.
Thus there is room for improvement and a need to provide a novel screwdriver bit.